Foaming materials are widely used in the daily life; weather-proof seals of cars, heat insulation layers, vibration proof substances on appliances, and soles of shoes are all made of such materials. The materials include ethylene vinyl acetate (EVA), styrene-butadiene rubber (SBR), natural rubber (NR), acrylonitrile-butadiene rubber (NBR), ethylene-propylene-nonconjugated diene rubber (EPDM), butadiene rubber (BR), polyethylene (PE), polystyrene (PS), poly vinyl chloride (PVC), polyurethane (PU), polypropylene (PP), chloroprene rubber (CR), isoprene rubber (IR) and styrene-butadiene-styrene elastomer (SBS) which is also the well-known primary material of soles of shoes. Foaming materials contain a large amount of bubbles, in other words, foaming rubbers or plastics can be seen as composite materials filled with gases. And like other synthetic materials, characteristics of foaming rubbers or plastics can be altered to meet different needs by modifying compositions or processing procedures thereof.
Typically, additives are added into a polymer matrix which is the main part of foaming materials and dominates basic characteristics of such materials. The primary purposes of additives are to improve processing properties of the polymer and to reduce the cost. To meet various needs, the composition of the additives and the polymer matrix can be altered.
In the traditional thermosetting rubber, NR, SBR and CR are well-known close-cell foaming materials. However, they cause air pollution during the manufacturing process by producing SOx during the vulcanization process. NR, SBR and CR are not only hazardous to environment but also impossible to recycle because they are all thermosetting rubbers. These disadvantages limit applications of such materials.
On the other hand, “ethylene-propylene copolymer” and “ethylene-α-olefin copolymer” are used in the close-cell foaming material. Nevertheless, they are not soft in touch and deform easily under compression. Usually the properties can be improved by adding “polyolefinic rubbers” at specific ratios into these copolymers. But in some circumstances, compatibilities between the copolymers and the rubbers are poor. These copolymers also are not easily processed with processing oils, and thus they are not suitable for making a soft foaming material with low specific gravity.
A flexible thermoplastic article comprising from about 2 to about 15 percent by weight styrene-elastomer block copolymer, from about 60 to about 90 percent by weight polyolefin, from about 4 to about 15 percent by weight plasticizer oil and from about 0.05 to about 3 percent by weight additive was proposed in U.S. Pat. No. 6,653,360. And the styrenic-elastomer block copolymer further comprises SEBS. The flexibility of the article was improved by adding SEBS having excellent compatibility with the plasticizer oil. Nonetheless, the contents of the plasticizer oil and the SEBS are low, so the hardness and specific gravity of the flexible thermoplastic article are still high. And it is not suitable for mass production since two processing steps are needed for manufacturing such article.
In view of needs for a novel material which is environmentally friendly, flexible and easy to be produced, an object of the present invention is to provide a thermoplastic elastomer foaming material which is soft, recyclable and mass producible.